The present invention relates to a frame strip system, and more particularly to a method of and apparatus for stripping a returned transmission frame of a communication apparatus connected to a ring-type medium.
In a ring-type Local Area Network (LAN) with a plurality of stations being connected to the same medium, it is necessary for a transmission station to strip reliably and by all means a frame transmitted onto LAN. According to a known transmission frame strip method, whether or not a transmission frame circulated on the ring once is a returned "my" transmission frame, is judged by searching a source address (SA) included in the receive frame. Namely, the transmission station checks a match of my address (MA) with the source address (SA) to confirm that the presently received frame is my transmission frame that has circulated the ring once and returned thereto, and the received frame is stripped.
In a network system having a plurality of LANs connected via a forward apparatus (bridges) and having a single LAN structure in a logical sense, in order to ensure transparency of a frame passing from one LAN to another LAN, transparency of the source address (SA) of the forward frame should be guaranteed. Specifically, the source address of a frame received from one LAN is forwarded to another LAN without referencing it to my address (MA) assigned to the forward apparatus. As described above, it becomes therefore impossible to discriminate the returned transmission frame by checking a match of MA with SA. According to a related technique, this problem is solved by a method of stripping received frames same in number as transmission frames, as disclosed, e.g., JP-A-1-206755 laid open on Aug. 18, 1989.
This related technique however does not consider how an error frame and a special frame are processed in stripping returned transmission frames, to thereby leave a possibility of infinite frame circulation.
There is known another related technique for stripping a frame, as disclosed in Digital Equipment Corp. (DEC), Henry Yang, K. K. Ramakrishnan and Bill Hawe, "Proposal on Frame Stripping for Bridges in FDDI", Jun. 16, 1989.